U.S. patent number 5,869,836 [Application Number 08/897,173] was granted by the patent office on 1999-02-09 for scintillation detector with sleeved crystal boot.
This patent grant is currently assigned to Saint-Gobain Industrial Ceramics, Inc.. Invention is credited to Chris W. Linden, Jeffrey R. Lutz, William D. Sekela.
United States Patent |
5,869,836 |
Linden , et al. |
February 9, 1999 |
Scintillation detector with sleeved crystal boot
Abstract
A scintillation detector (10) includes a scintillation crystal
(14) and a shock absorbing member (76) circumscribing the crystal
(14). A sleeve (98) circumscribes the shock absorbing member (76)
which, in turn, is circumscribed by a housing (12). The sleeve (98)
provides for substantial controlled radial loading on the crystal
(14). A method of manufacturing the detector (10) includes placing
the crystal (14) and shock absorbing member (76) into the sleeve
(98), compressing the sleeve 98 and inserting the compressed sleeve
(98) into the housing (12) such that the sleeve (98) substantially
maintains its compression. The radial stiffness causes vibration
induced counts to occur at an excitation frequency which is above
the operational bandwidth of the radiation measurements, thereby
excluding vibration induced counts for radiation measurements.
Inventors: |
Linden; Chris W. (North
Ridgeville, OH), Lutz; Jeffrey R. (Brunswick, OH),
Sekela; William D. (Aurora, OH) |
Assignee: |
Saint-Gobain Industrial Ceramics,
Inc. (Worcester, MA)
|
Family
ID: |
21831469 |
Appl.
No.: |
08/897,173 |
Filed: |
July 18, 1997 |
Current U.S.
Class: |
250/361R;
250/256 |
Current CPC
Class: |
G01T
1/2002 (20130101); G01V 5/04 (20130101) |
Current International
Class: |
G01V
5/00 (20060101); G01V 5/04 (20060101); G01T
1/20 (20060101); G01T 1/00 (20060101); G01T
001/202 () |
Field of
Search: |
;250/361R,367,483.1,256 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0 402 035 A1 |
|
Dec 1990 |
|
EP |
|
WO 95/23983 |
|
Sep 1995 |
|
WO |
|
Primary Examiner: Hannaher; Constantine
Assistant Examiner: Jiron; Darren M.
Attorney, Agent or Firm: Ulbrich; Volker R.
Claims
What is claimed is:
1. A scintillation detector comprising a scintillation crystal, a
resiliently compressed shock absorbing member circumscribing the
crystal, a sleeve circumscribing the shock absorbing member, and a
housing having a casing wall circumscribing the sleeve and radially
constraining the sleeve against radial expansion forces exerted
thereon by the resiliently compressed shock absorbing member.
2. A scintillation detector as set forth in claim 1, comprising a
reflector interposed between the crystal and the shock absorbing
member, wherein the reflector substantially surrounds the
crystal.
3. A scintillation detector as set forth in claim 1, wherein the
crystal, the shock absorbing member, the sleeve and the casing wall
are each cylindrical and concentric.
4. A scintillation detector as set forth in claim 1, wherein the
shock absorbing member comprises a material having a surface shape
selected from the group consisting of a uniform surface, axially
extending ribs, circumferentially extending ribs, outwardly
directed protrusions, inwardly directed protrusions, and inwardly
and outwardly directed protrusions.
5. A scintillation detector as set forth in claim 1, wherein the
shock absorbing member is a cylindrical boot.
6. A scintillation detector as set forth in claim 1, wherein the
shock absorbing member is made of an elastomeric material.
7. A scintillation detector as set froth in claim 1, wherein the
sleeve is substantially cylindrical and has a longitudinally
extending slit which allows the sleeve to radially expand or
contract.
8. A scintillation detector comprising:
a scintillation detector subassembly comprising a scintillation
crystal, a resiliently compressed shock absorber around the
crystal, and a relatively incompressible and relatively rigid
sleeve around the compressed shock absorber; and
a housing surrounding the subassembly, the subassembly being
maintained by the housing in a compressed state and under a radial
load by radial expansion forces exerted by the compressed shock
absorber.
9. A method of manufacturing a scintillation detector, comprising
the steps of placing a scintillation crystal within a resiliently
compressible shock absorbing member, placing the shock absorbing
member within a sleeve having an outside diameter, radially
compressing the sleeve around the shock absorbing member and
crystal to a reduced outside diameter whereat the shock absorbing
member is resiliently radially compressed, and inserting the
compressed sleeve into a housing having an inside diameter that is
smaller than the outside diameter of the uncompressed sleeve.
10. A method as set forth in claim 9, wherein the radially
compressing step further comprises positioning a radial compressing
device on the sleeve at a first location adjacent but spaced apart
from an end of the sleeve to be first inserted into the housing for
compressing the sleeve in the vicinity thereof to a reduced
diameter.
11. A method as set forth in claim 10, wherein said inserting step
comprises inserting a portion of the crystal, shock absorbing
member and sleeve which is compressed to the reduced outside
diameter into the housing by the radial compressing device,
repositioning the radial compressing device on the sleeve at a
second location adjacent and axially spaced apart from the first
location for compressing the sleeve in the vicinity thereof to a
reduced diameter, and repeating the insertion and repositioning
steps until the sleeve is inserted into the housing.
12. A method of measuring radiation, comprising the steps of using
a scintillation detector having a scintillation crystal constrained
within a scintillation crystal housing such that the radial
stiffness thereof is sufficiently great so as to make vibration
induced counts occur above an environmental excitation threshold
frequency, and operating the scintillation detector in an
operational environment having a dynamic bandwidth below the
environmental excitation threshold frequency, thereby substantially
eliminating vibration induced counts in radiation based
measurements.
13. A method as set forth in claim 12, comprising the step of using
a light sensing device to convert photons received from the
scintillation detector into electrical signals, wherein the
electrical signals substantially represent the radiation being
measured.
Description
This application claims the benefit of U.S. Provisional Application
No. 60/026,377 filed Sep. 20, 1996.
FIELD OF THE INVENTION
The invention herein described relates generally to a scintillation
detector and method for performing radiation-based measurements,
and to a method of manufacturing a scintillation detector. The
scintillation detector and method are particularly useful for
borehole logging applications, but may, however, have use in other
applications, particularly those plagued by vibration induced
counts intermixed with radiation induced counts.
BACKGROUND OF THE INVENTION
Scintillation detectors have been employed in the oil and gas
industry for well logging. These detectors have used thallium
activated sodium iodide crystals that are effective in detecting
gamma rays. The crystals are enclosed in tubes or casings to form a
crystal package. The crystal package has an optical window at one
end of the casing which permits radiation induced scintillation
light to pass out of the crystal package for measurement by a light
sensing device such as a photomultiplier tube coupled to the
crystal package. The photomultiplier tube converts the light
photons emitted from the crystal into electrical pulses that are
shaped and digitized by associated electronics. Pulses that exceed
a threshold level are registered as counts that may be transmitted
"uphole" to analyzing equipment or stored locally.
The ability to detect gamma rays makes it possible to analyze rock
strata surrounding the bore holes, as by measuring the gamma rays
coming from naturally occurring radioisotopes in down-hole shales
which bound hydrocarbon reservoirs. Today, a common practice is to
make measurements while drilling (MWD). For MWD applications, the
detector must be capable of withstanding high temperatures and also
must have high shock resistance. At the same time, there is a need
to maintain performance specifications.
A problem associated with MWD applications is that the detector
will report a higher than an actual count rate if the scintillation
crystal package produces vibration induced light pulses. The harsh
shock and vibration conditions the detectors encounter during
drilling can cause a crystal package to emit spurious light pulses
in addition to gamma ray induced light pulses. That is, the
detector output will be composed of radiation induced counts and
vibration induced counts. Heretofore, the detector electronics
could not distinguish the vibration induced counts from the genuine
gamma counts, whereby the detector reports a higher than actual
count rate. The problem is more severe when detecting low level
radiation events while the detector is being subjected to a very
severe dynamic operational environment.
Some prior art electronic solutions have attempted to filter out
vibration induced counts by discriminating on the basis of the
pulse shape and/or the signal decay time. These techniques,
however, have proven unreliable.
SUMMARY OF THE INVENTION
The present invention provides a "hardware" solution to the
aforesaid problem. According to one primary aspect of the
invention, components of a radiation detector assembly are
rigidified or stiffened to move the resonant frequencies of
vibration induced counts from the detector assembly above a
threshold frequency (i.e., the upper limit of the operational
dynamic disturbance bandwidth). This is accomplished by loading the
scintillation crystal both axially and radially such that the
several different scintillator rigid body resonant frequencies are
above the threshold frequency. Thus, the crystal is loaded within a
housing to provide sufficient stiffness such that the operational
dynamic bandwidth of the detector application falls below the
resonant frequency of vibration induced counts. Therefore, within
that environment, vibration induced counts will either not occur or
will have a magnitude that falls below an amplitude threshold and
will therefore be ignored.
According to a preferred embodiment of the invention, axial loading
of the scintillation crystal may be effected in a well known or
other suitable manner, while radial loading is accomplished through
the novel use of a sleeve split along its axial length such that it
can be radially expanded and contracted around a resiliently
compressible boot or other shock absorbing member circumscribing
the scintillation crystal. The split sleeve is assembled around the
boot and scintillation crystal to form a subassembly insertable
into a housing, such as a tube or casing preferably made of metal.
The housing is internally dimensioned such that the boot is
maintained in radial compression for application of a radial
compression load on the scintillation crystal. As will be
appreciated, the wall thickness of the split sleeve may be selected
as desired to provide a predetermined amount of radial loading and
thus stiffness. The radial stiffness, along with the axial
stiffness, may be selected to impart sufficient rigidity to the
detector assembly such that the several different resonant
frequencies associated with different vibration modes of the
detector assembly, which will produce vibration induced counts if
excited, will occur above the operational threshold frequency for
the given application.
The sleeve preferably is made of metal having a coefficient of
friction with the housing that is substantially less than the
coefficient of friction between the resiliently compressible boot
and a housing for the scintillation crystal and boot, thereby
providing for reduced frictional resistance during insertion of the
crystal/boot/split sleeve subassembly into the housing which is
internally dimensioned less than the unloaded radial dimension of
the subassembly. The split sleeve preferably has sufficient
stiffness or rigidity to enable compression of the boot over an
axial extent thereof extending axially beyond the location at which
the sleeve is compressed by a compression ring, clamp or other
suitable member used to facilitate insertion of the crystal, boot
and sleeve subassembly into the housing. Accordingly, an end of the
subassembly may be radially compressed by the compression ring
surrounding the split sleeve at a point spaced from such end so as
to permit insertion of such end into the housing with less force
than would be necessary without the split ring while still
providing the desired radial loading.
Thus, the present invention additionally provides an improved
method of assembling a detector assembly. The improved method
enables the manufacture of a detector assembly with a radial
loading of the scintillation crystal substantially greater than
that heretofore provided in similar detector assemblies. In
addition, the improved method employing a split sleeve can be used
to facilitate manufacture of detector assemblies regardless of the
extent of radial loading.
Therefore, according to the invention, a scintillation detector
comprises a scintillation crystal, a resiliently compressed shock
absorbing member circumscribing the crystal, a sleeve
circumscribing the shock absorbing member, and a housing having a
casing wall circumscribing the sleeve. A reflector may be
interposed between the crystal and shock absorbing member to
provide optimal collection of radiation induced counts. The sleeve
may also have a longitudinally extending gap in the wall thereof to
provide for radial compression, thereby providing for substantial,
uniform and controlled radial loading on the crystal.
According to another aspect of the invention, a scintillation
detector comprises a scintillation detector subassembly including a
scintillation crystal, a resiliently compressed protection means
around the crystal, and compressing means around the compressed
protection means. A housing circumscribes the subassembly such that
the subassembly remains in a compressed state, thereby providing
uniform, controlled radial loading on the crystal.
According to yet another aspect of the invention, a method of
manufacturing a scintillation detector includes placing a
scintillation crystal within a resiliently compressible shock
absorbing member and placing the shock absorbing member in a
sleeve. The sleeve and shock absorbing member are radially
compressed and inserted into a housing which substantially
maintains the radial compression, thereby achieving an interference
fit between the sleeve and the housing and controlled uniform
radial loading along the crystal.
According to a further aspect of the invention, a method of
measuring radiation includes the steps of using a scintillation
detector having a scintillation crystal loaded within a
scintillation crystal housing such that the stiffness on the
crystal is sufficiently great such that any vibration induced
counts of sufficient amplitude to be recorded as an event occur
above a threshold frequency, i.e., above the upper limit of the
operational dynamic disturbance bandwidth to which the detector is
exposed during radiation measurement. That is, the detector is used
in an environment that has a dynamic bandwidth below the threshold
frequency, thereby substantially eliminating vibration induced
counts in radiation based measurements.
According to a still further aspect of the invention, a method for
making radiation based measurements in a high vibration environment
includes positioning a scintillation detector having a
scintillation crystal in a high vibration environment for
interaction with incident radiation, wherein the scintillation
crystal has sufficient stiffness such that vibration induced and
recordable photons are not excited by the crystal's environmental
dynamic conditions. A light sensing device receives emitted photons
from the scintillation crystal and converts the photons into
electrical signals, wherein the electrical signals substantially
represent the radiation being measured.
The invention comprises the foregoing and other features
hereinafter fully described and particularly pointed out in the
claims, the following description and the annexed drawings setting
forth in detail a certain illustrative embodiment of the invention,
this being indicative, however, of one of the various ways in which
the principles of the invention may be employed.
BRIEF DESCRIPTION OF THE DRAWINGS
In the annexed drawings:
FIG. 1 is a fragmentary longitudinal sectional view of a
scintillation detector according to the invention;
FIG. 2 is an exploded view of components of the scintillation
detector according to the invention; and
FIG. 3 is a perspective diagram illustrating a scintillation
detector subassembly being compressed and inserted into a
housing.
DETAILED DESCRIPTION OF THE INVENTION
The problem of vibration induced counts associated with MWD
applications is solved by appreciating that vibration induced
counts are a function of the dynamic rigid body vibration modes of
the crystal. By increasing the axial and radial stiffness on the
crystal within the detector, the frequency of excitation needed to
effect recordable vibration induced counts is also increased. In
the past, high axial loading could be relatively easily
accomplished. However, it was difficult, if not impossible, to
attain the necessary radial loading using known assembly
techniques. The radial dimension of the uncompressed subassembly
needed to attain such high radial loads would be such that attempts
to insert the subassembly into the housing would not be possible or
would cause damage to the boot. According to the present invention,
high radial loading can be accomplished by placing the boot and
crystal into a sleeve which is then radially compressed and
inserted into the detector housing. The frictional force between
the sleeve and housing is substantially less than that between the
boot and housing, thereby allowing a greater radial loading to be
accomplished during assembly without damage to the boot. The
increased radial stiffness causes vibration induced counts to occur
at higher excitation frequencies, such as at frequencies above a
threshold frequency, i.e., above the operational dynamic bandwidth
of the detector. Accordingly, the detector will not be exposed to
frequencies high enough to excite the vibratory modes of the
detector assembly, whereby the detector will not produce vibration
induced counts. Further, to the extent some white noise is present
in an MWD application, the magnitude of any vibration induced
counts are not sufficiently large to be tallied as a count, i.e,
are not of sufficient amplitude to be counted as a recordable
event.
The present invention provides an improved scintillation detector
assembly having a scintillation crystal circumscribed by a
resiliently compressible shock absorbing member. A sleeve
circumscribes the shock absorbing member and is compressible or
deformable to thereby apply a substantial radial loading on the
crystal that is uniform and controllable. The compressed or
deformed sleeve is circumscribed by a casing wall of a housing that
substantially maintains the radial loading on the crystal. The
stiffness imparted to the detector assembly is sufficient to not
cause vibration induced photon emissions in the intended
operational excitation bandwidth, thereby eliminating vibration
induced counts in radiation based measurements.
The sleeve may be composed of a material that has a substantially
lower coefficient of friction with the housing than the shock
absorbing member, thereby allowing a greater radial loading to be
achieved on the crystal due to the reduced frictional forces
encountered during insertion of the crystal into a housing.
Further, the sleeve may have a longitudinal gap to readily provide
for radial compression or may be deformable, crimped or fluted to
effectuate substantial radial loading on the crystal.
Referring now in detail to the drawings, FIG. 1 illustrates an
exemplary and preferred scintillation detector 10 according to the
present invention. The detector 10 comprises a housing 12
encapsulating a scintillation crystal 14. The crystal may be, for
example, a thallium-activated sodium iodide crystal as in the
illustrated embodiment. The crystal 14 has a cylindrical surface 16
and flat end faces 18 and 20, the surface finish of which may be
sanded, polished, ground, etc., as desired.
The housing 12 includes a tubular metal casing 22 which preferably
is cylindrical like the crystal 14 as in the present case. The
casing 22 is closed at its rear end by a back cap 24 and at its
front end by an optical window 26. The optical window 26 should be
made of a material transmissive to scintillation light given off by
the scintillation crystal 14. In the illustrated embodiment, the
optical window 26 is made of crown glass. The casing 22 and back
cap 24 preferably are made of stainless steel or aluminum, as is
conventional. The back cap 24 is joined to the rear end of the
casing 22 by a vacuum type peripheral weld. As seen at the left in
FIG. 1, the cylindrical wall 28 of the casing in interiorly
recessed to form a welding flange 30 which defines a closed fitting
pocket for receipt of the back cap 24. The back cap 24 has, opening
to its outer side, an annular groove 34 spaced slightly inwardly
from its circumferential edge to form a thin annular welding flange
36 and a reduced narrow thickness connecting web 38. Welding is
effected at the outer ends of the juxtaposed thin welding flanges
30 and 36 and the reduced thickness of the connecting web 38
further reduces welding heat conduction away from the welding
flanges to permit formation of a desired weld.
The back cap 24 and crystal 14 have sandwiched therebetween, going
from left to right in FIG. 1, a spring 40, a backing plate 42, a
cushion pad 44 and an end reflector 46. The spring 40, or other
suitable resilient biasing means, functions to axially load the
crystal and bias it towards the optical window 26, as is
conventional. The spring 40 is a stack of wave springs disposed
crest-to-crest as shown. Other springs that may be used include
coil springs, resilient pads, and the like.
The backing plate 42 functions to spread the spring force across
the transverse area of the cushion pad 44 for substantially uniform
application of pressure and axial loading to the rear face 18 of
the crystal 14. The cushion pad 44 is made of a resilient material
and preferably a silicone rubber (elastomer) to which a reflecting
material such as aluminum oxide powder may be added. The thickness
of the cushion pad may range, for example, from about 0.06 to 0.30
inches for most conventional size crystals ranging in diameter from
about 0.25 to 3.0 inches and in length, for example, from about 0.5
to 15 inches.
The cushion pad 44 backs up against the end reflector 46 which is
formed by at least one sheet of a white thin porous unscintered
PTFE material. Being porous, air or gas can escape from between the
reflector film and crystal face to avoid pockets of trapped air or
gas. Such pockets are usually undesirable since trapped air or gas
could prevent the reflector 46 from being pushed by the cushion pad
44 flat against the rear end face 18 of the crystal 14 and thus
have a negative impact on reflectability at the reflector-crystal
interface. It also will be appreciated that the resilient pad 44
will conform to the rear end face 18 of the crystal 14 should the
rear end face 18 not be perfectly flat. The reflector material may
be a 0.010 inch thick, 1.5 gm/cc film which is wrapped at least
once around the crystal and possibly two or a few times as desired.
The end reflector 46 may alternatively be a tin foil disk which
conforms to the surface of the crystal end face 18 and provides
suitable reflectance to thereby direct scintillation light toward
the optical window 26.
As indicated above, the spring 40 resiliently pushes the crystal 14
towards the optical window 26 to maintain an optical coupling
between the front end face 20 of the crystal 14 and the interface
of the optical window 26. In the illustrated embodiment, the
optical coupling is effectuated by a layer 52 (or interface pad) of
suitable optical coupling material and may be a silicone rubber pad
sandwiched between the crystal 14 and the optical window 26. The
interface pad 52 may be preformed prior to assembly of the detector
10 and is not bonded to the crystal 14 and/or optical window 26
such that the result is a contact only interface between the
interface pad 52 and the crystal 14 and/or optical window 26. An
exemplary material for the interface pad 52 is a transparent
silicone elastomer. The thickness of the interface pad 52 may range
from about 0.06 to 0.30 inch for most conventional size crystals
ranging in diameter from about 0.25 to 3.0 inches and in length
from about 0.5 to 15 inches.
As seen at the right in FIG. 1, the optical window 26 is retained
in the casing 22 by an annular lip 58 at the front end of the
casing 22. The lip 58 protrudes radially inwardly from the casing
wall 28 and defines an opening having a diameter less than the
diameter of the window 26. The lip 58 has an axially inner beveled
surface 60 and the optical window 26 has a corresponding beveled,
axially outer, circumferential edge surface 62 which seats against
the beveled surface 60. The mating beveled surfaces are
hermetically sealed by a high temperature solder such as 95/5 or
90/10 lead/tin solder. The solder also aids in restraining the
window 26 against axial push-out, although its primary function is
to effect a high temperature seal. As is apparent from the
foregoing, the window 26 is axially trapped between the lip 58 and
the crystal 14 and is radially constrained by the casing wall 22.
To permit wetting of the glass 26 by the solder, the sealing edge
surfaces of the window 26 may have applied thereto a metalized
coating such as platinum.
The beveled lip surface 60 may forwardly terminate at a relatively
small diameter cylindrical surface 66 and rearwardly at a
relatively larger diameter cylindrical surface 68. The cylindrical
surface 68 closely surrounds the axially inner portion of the
optical window 26 and extends axially inwardly to a slightly larger
diameter cylindrical surface 70 which extends axially to the flange
30 at the rear end of the casing 22. The axial interface of the
window 26 is aligned with the annular shoulder formed between the
cylindrical surfaces 68 and 70.
Between the optical window 26 and the end reflector 46, the crystal
14 is surrounded by a layer 74 of reflecting material which in turn
is surrounded by a shock absorbing boot 76. The layer 74 of
reflecting material preferably is the above-mentioned white thin
porous PTFE material. As noted above, air or gas that might
otherwise be trapped between the reflector 46 and the crystal 14
can escape through the porous reflector media 74. The porous PTFE
film 74 is tightly wrapped around the crystal 14 and is generally
self-adhering to the cylindrical surface 16 of the crystal 14.
The shock absorbing boot 76 closely surrounds and preferably grips
the reflector layer 74 to aid in holding the PTFE reflector film 74
tight against the crystal 14. As shown, the boot 76 is preferably
cylindrical and concentric with both the crystal 14 and the casing
22. The boot 76 is made of resiliently compressible material and
preferably is a silicone rubber, elastomer, or silicone elastomer,
the latter being a fast setting silicone elastomer. Preferably, the
silicone elastomer does not include any fillers such as Al.sub.2
O.sub.3 powder that may degrade performance.
Alternatively, the shock absorbing boot 76 may comprise any member
that provides a shock absorbing function about the circumference
and length of the crystal. The member 76 may have a uniform inner
surface 77 and outer surface 78 or may have ribs extending axially
or circumferentially on either the inner surface 77 or the outer
surface 78. In other alternative embodiments, the shock absorbing
member 76 may have dimples or geometrically shaped protrusions on
either the inner surface 77, the outer surface 78, or both.
A locating ring 90 extends from the front end of the boot 76 to the
optical window 26. The locating ring 90 has an axially inner end
portion 92 surrounding the crystal 14 and an axially outer end
portion 94 surrounding the interface pad 52. At the intersection of
the interior surfaces of the axially inner and outer portions there
is a shoulder 96 which functions to locate the locating ring 90 on
the crystal 14 during assembly. The locating ring 90 is made of
resilient material and preferably a silicone rubber to which
Al.sub.2 O.sub.3 powder may be added for reflection purposes. The
locating ring 90 functions to hold and center the circular
interface pad 52 during assembly of the detector 10.
Interposed between the casing 22 and the boot 76 is a sleeve 98
which extends longitudinally from the optical window 26 nearly to
the back cap 24. The sleeve 98, when circumscribing the boot 76 and
crystal 14 in a substantially uncompressed state, has an outside
diameter that exceeds the inside diameter of the tubular metal
casing 22. Therefore, to insert the sleeve 98 into the casing 22,
the sleeve 98 must be compressed, thereby causing the boot 76, made
of resilient material, to radially compress against the crystal 14,
thereby radially loading the crystal 14. Preferably the sleeve 98
is metal, for example, stainless steel. Alternatively, however, the
sleeve 98 according to one broad aspect of the invention may be
composed of any material that has a lower coefficient of friction
with the casing 22 than does the boot 76 with the casing 22.
The sleeve 98 must therefore be radially compressible to effectuate
substantial radial compression of the boot 76 against the crystal
14. In a preferred embodiment, the sleeve 98 is slotted along its
longitudinal length, thereby providing a longitudinally extending
gap 99. The longitudinally extending gap 99 may vary between a
substantial width, when the boot 76 resides within the sleeve 98
without any externally applied compression, and almost no
appreciable width, when the sleeve 98 and boot 76 are under a
substantial radial compressive force when inserting the sleeve 98
and boot 76 into the casing 22. Under such compressive forces the
longitudinal edges of the slotted sleeve 98 approach and may come
into physical contact with one another causing the outside diameter
of the sleeve 98 to be reduced. A visual example of the slotted
sleeve 98 and the gap 99 is illustrated in FIG. 2 which will be
discussed infra.
Alternatively, the sleeve 98 may be compressible in other ways. For
example, the sleeve 98 may be a cylinder or substantially
cylindrical and formed of a radially flexible material which
sufficiently deforms under radial compressive forces to fit within
the casing 22 and thereby radially load the crystal 14 within the
boot 76. In another alternative, the sleeve 98 may be fluted or
crimped to allow for radial compression of the sleeve 98 along its
axial length.
The sleeve 98 provides for uniform and controlled radial loading of
the crystal 14. The thickness of the sleeve 98 along its axial
length may be controlled with tight tolerances, thereby providing
for uniform radial loading along the crystal's entire length. To
increase or decrease the amount of radial loading, the sleeve 98
thickness may be varied, wherein a thicker sleeve increases the
radial loading on the crystal 14 and vice-versa. Since the
thickness of the sleeve 98 may be tightly controlled, so too can
the radial loading on the crystal 14, and thus the stiffness of the
crystal.
The sleeve 98 also facilitates assembly of a crystal-boot
subassembly into the casing. During insertion of the crystal-boot
subassembly into the casing 22, the sleeve 98 provides a
coefficient of friction between the sleeve 98 and the metal casing
22 which is substantially less than the coefficient of friction
between the boot 76 and the casing 22. This feature will be further
described in conjunction with FIG. 2.
FIG. 2 is an exploded perspective view illustrating a manner in
which the detector 10 may be assembled. After appropriately
wrapping the crystal 14 with the reflecting layer 74, the crystal
14 is inserted into the boot 76 and the boot 76 in the sleeve 98 to
form the a crystal-boot-sleeve subassembly. At this point, the
outside diameter of the sleeve 98, with the boot in an uncompressed
state, will be greater than the inside diameter of the metal casing
22.
Therefore, to insert the sleeve 98 into the casing 22, a radial
compression force is applied to the sleeve 98 at an end first to be
inserted into the casing to compress the sleeve 98 sufficiently to
enable insertion of the subassembly into the casing 22 preferably
with the use of a forcing mechanism 100. The forcing mechanism 100,
for example, may consist of a hydraulic ram or push rod 102 coupled
to a conventional control apparatus 103 for pushing the
crystal-boot-sleeve subassembly into the casing 22. After a first
incremental insertion of the subassembly into the casing, the
radial compression force is then re-applied to the sleeve 98 at a
location spaced a short distance from the sleeve/casing interface
to facilitate further insertion of the sleeve 98 into the casing
22. The steps are then repeated until the sleeve 98 entirely
resides within the casing 22.
The incremental compression preferably is accomplished with a
radial clamp 104, for example a compression ring, secured to the
sleeve 98 as illustrated in FIG. 3. Each time the radial clamp 104
is secured to the sleeve 98, a length of the sleeve 98 will be
sufficiently compressed for insertion into the casing 22. The
length of the sleeve 98 available for insertion is a function of
the axial rigidity of the sleeve 98. For example, a sleeve 98
having very little axial rigidity would have a small length
available for insertion while a sleeve 98 having substantial axial
rigidity will have a longer length available for insertion. The
axial rigidity of the sleeve 98 will therefore necessarily impact
the location at which the radial clamp 104 is applied to the sleeve
98. An axial rigidity is selected such that a length of 0.25 inch
may be inserted into the casing 22 at one time. An axial rigidity
may be selected to allow, for example, 0.5 inch insertion to be
effectuated or alternatively allow 1.0 inch or greater insertion
before the need to reposition the radial clamp, or other
compressing device. Accordingly, the crystal-boot-sleeve
subassembly may be progressively inserted at increments ranging
from about 0.25 inch to 1.0 inch, or more.
The insertion process of FIG. 2 benefits from the metal-to-metal
interface between the sleeve 98 and casing 22 which substantially
reduces the coefficient of friction relative to metal-to-boot
interface which would exist without the sleeve 98. The boot 76
typically will have a higher coefficient of friction with the
casing 22 than the sleeve 98. Therefore, for a desired radial
loading (obtained by compressing the sleeve-boot-sleeve subassembly
by a desired percentage of its uncompressed dimension), it will be
substantially more difficult to insert the crystal 14 and boot 76
into the metal casing 22 without the sleeve 98. Further, for large
radial loadings, such as those necessary to obtain the above
described shifting of the resonant frequencies of the detector
assembly for MWD applications, it is extremely difficult if not
impossible to insert, while maintaining the integrity thereof, a
crystal-boot subassembly into the casing 22 without the sleeve 98
due to the substantial coefficient of friction between the boot 76
and casing 22. At such radial loading levels, insertion of a
crystal-boot subassembly (sans sleeve) directly into the casing 22
undesirably would cause damage to the boot 76, thereby
substantially reducing the boot's functionality as a shock absorber
for the high vibration environment experienced by the detector 10.
The use of the sleeve 98 enables a substantially higher radial
compression force to be achieved while maintaining the integrity of
the boot.
Although the invention has been shown and described with respect to
certain preferred embodiments, it is evident that equivalent
alternation and modifications will occur to others skilled in the
art upon the reading and understanding of this specification and
the annexed drawings. In particular regard to the various functions
performed by the above described elements (components, assemblies,
devices, compositions, etc.), the terms, including a reference to a
"means" used to describe such elements are intended to correspond,
unless otherwise indicated, to any element which performs the
specified function of the described element (i.e., that is
functionally equivalent), even through not structurally equivalent
to the disclosed structure.
* * * * *